Catalysts in which titanium compounds are supported on magnesium halides have been hitherto known as those used for preparing homopolymers of ethylene or .alpha.-olefins or those used for preparing ethylene/.alpha.-olefin copolymers. One example of the olefin polymerization catalysts is a catalyst comprising a solid titanium catalyst component, which comprises magnesium, titanium, halogen and an electron donor, and an organometallic compound catalyst component.
For preparing the solid titanium catalyst component used as the olefin polymerization catalyst component, many proposals have been made so far. For example, it is known to contact a halogen-containing liquid magnesium compound with a liquid titanium compound to produce a solid product.
In the conventional processes for preparing solid titanium catalyst component, anhydrous magnesium chloride for industrial use and/or reagent use has been employed as the magnesium halide compound. However, the anhydrous magnesium chloride for industrial use and/or reagent use has problems in cost, stability of supply and the like. Hence, substitution of hydrous magnesium chloride available at low cost and stably, e.g., hydrous magnesium chloride produced from seawater, that is generally called "bittern", will be of great industrial value.
However, a solution of the anhydrous magnesium chloride obtained from the bittern contains calcium as impurity. When the solution of the anhydrous magnesium chloride containing calcium as impurity is used to prepare a solid titanium catalyst component, there is such a problem that the calcium as impurity decreases the polymerization activity of the resulting catalyst.
Under such circumstances as mentioned above, the present inventors have studied processes for preparing anhydrous magnesium halides containing no calcium as impurity from hydrous magnesium halides (e.g., hydrous magnesium chloride) containing at least calcium as impurity. As a result, they have found that an anhydrous magnesium halide solution containing no calcium as impurity can be obtained by a process comprising distilling water off from an oxygen-containing organic solvent solution of hydrous magnesium halide containing at least calcium as impurity to prepare an oxygen-containing organic solvent solution of anhydrous magnesium halide, wherein in any of stage of the step of preparing the oxygen-containing organic solvent solution of hydrous magnesium halide to the step of preparing the oxygen-containing organic solvent solution of anhydrous magnesium halide, a potassium compound is added to contact the calcium (calcium compound) with the potassium compound and thereby precipitate calcium (calcium compound) together with potassium (potassium compound), followed by removing them from the oxygen-containing organic solvent solution of anhydrous magnesium halide. The present inventors have further found that a solid titanium catalyst component of high activity can be prepared by the use of the anhydrous magnesium halide solution obtained by the above process.